Vitamin K has an emerging role in bone health. However, little is known about the vitamin K metabolism, particularly in regards to aging, and maintenance of bone mass. Such limited understanding about vitamin K metabolism currently impedes the ability to establish dietary recommendations for vitamin K, and interpret the results from recent clinical trials on vitamin K supplementation and bone health currently being conducted in primarily women of a narrow age group. The proposed study will be the first to assess the role of dietary and non-dietary factors that influence the response to measures of vitamin K status and bone turnover to vitamin K depletion and repletion in younger and older men and women. This study will also compare the absorption efficiency of vitamin K, relative to current vitamin K status. Men and women [21 younger (21-40y) and 21 older (60-80y)] will participate in a 65-d metabolic study, with a 5-d run-in period, followed by a 30 d dietary vitamin K restriction period (10 mu/ g/d), and ending with a 30 d dietary vitamin K supplementation period (500 mu g/d). Serial measurements of markers of vitamin K status (plasma phylloquinone, urinary Gla, serum %ucOC and plasma PIVKA-II) and bone turnover (serum osteocalcin and NTx) will provide information on their response to dietary manipulation of vitamin K for both age groups under identical controlled dietary conditions. A stable isotope tracer (deuterium-labeled vitamin K in collards) will be used to compare the absorption of vitamin K during a vitamin K-deplete state (21 d of dietary vitamin K depletion) to that of a vitamin K-replete state (21 d of dietary vitamin K repletion). Because vitamin K is transported in triglyceride-rich lipoproteins, which may vary among individuals due to differences in adiposity, measurement of body composition by DXA and plasma lipids (total and individual lipoproteins) will provide insight into the role of lipids in absorption and transport of vitamin K. The findings of the proposed study are critical for the interpretation of the epidemiologic and clinical data used to determine optimal intakes at which vitamin K may have a protective role in bone health.